Polyethylene terephthalate (polyester) is a thermoplastic polymer having high tensile strength, nearly equal to that of steel. Additionally, films of polyester are unusually resistant to tearing. Polyester has numerous applications in a variety of products, including fabrics, packaging films, recording tapes, and soft-drink bottles. Polyester is also a standard film base for photographic use and particularly for films that use a silver halide photographic emulsion.
Polyester with its strength and tear-resistance properties is a high-demand product. Limited production capability and availability of the raw materials for producing polyester further increase the demand for this product. Because of this high demand for polyester, especially in a climate of increased environmental awareness, there is a need for a method of recycling polyester into useable products. One major source of raw material is recovered polyester bottles. Another potential source is photographic film.
There are many types of photograph film, but all have the same general characteristics. Photographic film in general consists of a support layer, such as glass, a plastic sheet, or paper, coated with an emulsion layer, usually a suspension of silver halide crystals in gelatin. The emulsion layer provides the light-sensitive layer in which the picture is formed.
The composition of the film support layer is dependent upon the particular use of the film. For example, when improved dimensional stability is required, the film support may consist of one of a variety of polymeric materials, including polyester.
The emulsion layer is basically a suspension of silver halide crystals in gelatin, prepared by adding a solution of silver nitrite to a solution of alkali halides in gelatin. The emulsion is coated on the support layer and is set and dried according to the specific use for the film. Many films have more than one coating, including X-ray film which is coated on both sides. Before the emulsion is coated on films, a substratum or "sub" is applied to assure good adhesion of the gelatin layer.
X-ray film, a type of photographic film, is potentially an enormous source of recyclable polyester. The medical community alone generates several million pounds of spent X-ray film each year. The composition of X-ray film, however, has thus far precluded successful recycling incentives. As in other types of photographic film, X-ray film is typically formed of a polyester film base which supports the photographic silver halide emulsion layer. To promote adhesion, X-ray film often contains a sub binding layer of polyvinylidene chloride (PVDC), which adheres well to both the photographic emulsion and to the polyester base. In addition, a blue dye is often included in the X-ray film, apparently because some radiologists prefer the appearance and contrast the blue tint gives the developed X-ray images. Additionally, recovered polyester X-ray film is often found mixed with other films such as acetate.
Currently, the photographic emulsion alone can be successfully removed from the X-ray film by any one of a variety of known desilvering process. The PVDC and blue dyes, however, present an obstacle to the complete and successful recovery of the polyester base. When the desilvered polymeric base is recycled in the presence of the PVDC, the PVDC causes severe yellowing of the polyester in extrusion and can also cause corrosion of the extruding equipment. Further, the blue dye limits the polyester's use to products that are noncolorsensitive.
Because of these difficulties, most desilvered film is landfilled. If both the PVDC and the blue dye could be removed, the spent X-ray film would provide a major source of a usable polyester product, especially in the production of polyester fiber.
Various prior techniques for recovering polyester from photographic film have been attempted. For example, U.S. Pat. No. 3,647,422 to Wainer teaches the use of sodium hydroxide baths for the recovery of silver and polyester. This process melts the polyester base which can then be reworked into the desired shape. U.S. Pat. No. 3,928,253 to Thornton et al. also teaches a process for the recovery of polyester; aqueous monoethanolamine is used to strip the silver halide photographic emulsion from the film's polyester base. Another technique, taught in U.S. Pat. No. 4,150,977 to Phillips, involves the use of enzymes such as protease, amylase and lipase. The focus of all of these techniques, however, is the recovery of the silver component of the photographic emulsion. None of these patents address the problem caused by the presence of PVDC or the blue dye.
Other patents which teach the recovery of polyester from photographic film do recognize the problem the presence of PVDC raises. These patents, however, require complex processes for the recovery of PVDC, including the use of multiple solvents and multiple steps. Further, none address the additional problems raised by the presence of blue dye. For example, the technique taught in U.S. Pat. No. 3,873,314 to Woo et al. utilizes a polar aprotic solvent in a series of steps to dissolve the PVDC-containing polymer. U.S. Pat. No. 4,078,916 to Gerber et al. teaches the use of a combination of various solvents, particularly those of aromatic compounds. U.S. Pat. No. 4,602,046 to Buser teaches the use of a caustic alkali under condition of high shear to achieve separation.
These references are all examples of attempts made to recover polyester from photographic film. Various factors hinder the effectiveness or feasibility of any of these techniques for the complete recovery in the presence of both PVDC and blue dye. For example, all of these techniques are limited to recovery of polyester from silver or PVDC; none address the additional complications presented by the prevalent practice of incorporating dye compounds into the photographic film. Additionally, even where the PVDC problem is addressed, the references require complicated processes, involving multiple steps and multiple solvents. None provide for the removal of PVDC and blue dye in one step with one solvent. These limitations render these processes both ineffective and unfeasible for the recovery of polyester from photographic film.